I do it every brew now. IMHO the bitterness added is mellower. The flavor and aroma added during the steeping period(while you run your wort over them) is most likely boiled away after 60 minutes, but I guess you never know.
I mash hopped a couple of times, meaning I put 1.5 to 2 oz. of hops in the mash right after dough-in. I didn't experience any significant difference, I felt like it was a waste. Now FWH makes a big difference for me, I like the milder bitterness.